Fish and tap water: chlorine, chloramine, metals and conditioner failures

Quick answer

When a whole tank starts flashing, clamping its fins and gasping within hours of a water change, the cause is almost never a parasite. It is the water you just added.

Tap water is treated to be safe for people, and the things that make it safe for people are toxic to fish and to the nitrifying bacteria in your filter. Chlorine burns gill and skin surfaces on contact. Chloramine does the same and does not evaporate off. Copper leached from pipework kills invertebrates at levels no human would notice. Softened water is not soft in the way fish need.

Almost all of this is preventable, and most of it comes down to how the conditioner is dosed and what happened at the treatment works this week.

Netting fish out is the last resort in a water accident. Fixing the water is faster and does less damage than moving the animals.

• Letting water stand overnight removes free chlorine. It does not remove chloramine, which is what many supplies now use.
• Treat the water in the bucket, at the dose for that bucket, before it goes anywhere near the tank.
• Never use hot tap water for a water change. It carries more dissolved metals and often comes from a cylinder.
• Shrimp and snails die before fish show anything. Treat them as the early warning system.
• A chlorine event kills the filter as well as irritating the fish, so watch for an ammonia spike a few days later.

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Water companies periodically increase disinfectant levels in the mains, and the dose that worked all year suddenly does not.

Flushing after repairs, seasonal switching between chlorine and chloramine, and routine shock chlorination all raise the concentration in your tap without any announcement you are likely to see. If you dose conditioner by habit at the minimum rate, that is the week you lose fish. Dosing at the recommended rate for the full volume, not the minimum, and treating in a bucket rather than in the tank, is what protects against a supply you cannot control.
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Decide in 60 seconds

What is in the water and what it does

Chlorine.

Added as a disinfectant. It is a powerful oxidiser, and the surfaces it meets first in a fish are the gills and the mucus layer over the skin. It damages the thin lamellae where gas exchange happens, strips the protective slime coat, and irritates the eyes.

The visible result is a tank that flashes, rubs on decor, clamps its fins and breathes hard, sometimes within minutes. In a heavier exposure, fish become disoriented and die.

Free chlorine does gas off from standing water, which is where the old advice to leave a bucket overnight comes from.

Chloramine.

Many supplies now use chloramine instead, because it is more stable and persists further along the distribution network. That stability is precisely the problem for aquarists. Chloramine does not meaningfully evaporate from a standing bucket, and a supply that has switched to it will destroy a tank belonging to someone who has relied on standing water for years.

There is a second consequence. Chloramine is chlorine bonded to ammonia. A conditioner that breaks the bond neutralises the chlorine and leaves ammonia behind, which the biological filter then has to process. In a large water change on a lightly stocked or newly cycled tank, that ammonia can be measurable. Products that state they detoxify both chlorine and ammonia handle this; simple dechlorinators do not.

Ask your water company which they use. Many publish it, and some switch seasonally.

Copper and other metals.

Copper pipework releases copper into the water, and the amount depends on how long the water has stood in the pipe, the temperature, and the pH. Water drawn first thing in the morning, and water from the hot tap, carry the most.

Copper at concentrations entirely acceptable in drinking water is lethal to shrimp and snails and harmful to fish over time. This is one of the reasons invertebrates die first in a household water problem.

Never use the hot tap for a water change. It has been sitting in a cylinder or a heat exchanger, it carries more dissolved metal, and in some systems it contains additives that never enter the cold supply.

Softened water.

A domestic ion-exchange softener removes calcium and magnesium and puts sodium in their place. Fish, snails and plants need the calcium and magnesium. The water is soft in the sense that it will not scale a kettle, and useless in the sense a fishkeeper means.

If your house has a softener, find the untreated tap. Most installations leave the kitchen cold tap unsoftened for drinking, and that is the one to use.

Reverse osmosis water.

Strips almost everything, which is exactly why it is useful for keepers who want to build water to a specification. Used straight, it has no minerals and no buffering, and the pH is unstable. It must be remineralised with an appropriate product before it goes in a tank.

Well and borehole water.

No chlorine, but a different list: high nitrate in agricultural areas, iron, hydrogen sulphide, low oxygen, and dissolved carbon dioxide that raises pH sharply once the water is aerated. Water from a private supply should be tested before it is used for fish, not assumed to be clean because it is untreated.

Measure the container, measure the dose. Conditioner failures are almost always arithmetic failures.

How dechlorinator dosing actually goes wrong

Dosing the tank instead of the water.

Adding conditioner to the aquarium and then running tap water straight in means untreated water contacts fish and filter media before it mixes. In a small tank, or with a strong flow, that contact is enough. Treat the new water in its own container first.

Guessing the volume.

Bucket capacities are marked optimistically and tanks are rarely the volume on the box once substrate, rock and equipment are in. Fill the bucket to a marked line and dose for that line.

Dosing at the minimum rate.

Manufacturers give a rate that assumes a typical supply. Your supply is not always typical, and it is not typical at all during a shock chlorination. Dosing at the full recommended rate for the whole volume is inexpensive and covers the variation.

Old or badly stored product.

Conditioners degrade, especially once opened and left in a warm cupboard. If the bottle has been open for a long time and smells different from a new one, replace it.

Wrong product for the supply.

If your supply uses chloramine, you need a product that says it deals with chloramine and the ammonia released. A basic dechlorinator is not enough.

Inline units on a hose.

Convenient for large tanks and ponds, and they work well until the cartridge is exhausted. They have no warning light. Track the volume passed through and change the cartridge on a schedule rather than when something goes wrong.

Hosepipes.

A garden hose left in the sun grows biofilm, releases plasticisers, and delivers water at whatever temperature it has been sitting at. Use a hose reserved for the tank, flush it thoroughly before filling, and check the temperature.

Temperature.

Not a chemical issue but it belongs here, because it is the other half of most water change disasters. Match the temperature by hand before adding, and add slowly. A sudden drop stresses fish and can trigger disease outbreaks days later.

Recognising an exposure

The pattern is what identifies it: multiple fish affected at the same time, shortly after water entered the tank, with no new arrivals and no gradual build-up.

The signs, roughly in order of appearance:

Flashing and rubbing against decor and substrate, as the skin is irritated.

Rapid gill movement and gasping, as gas exchange is impaired.

Clamped fins and darting or erratic swimming.

Excess mucus, sometimes visible as a whitish film or as sheets of slime coming away.

Reddened gills, red streaking in the fins, and cloudy eyes.

Invertebrate deaths, often before any fish looks unwell.

Fish deaths, sometimes within hours, sometimes over the following day.

If you see this, act on the water immediately. Add a full dose of conditioner for the whole tank volume, increase aeration hard, turn off the lights, stop feeding, and do not add anything else. Do not attempt another water change from the same tap until you understand what happened.

Intact, glossy skin with an even slime coat. Patchy dullness or sheets of mucus coming away is chemical damage, not a parasite.

The delayed problem: your filter

Nitrifying bacteria are more sensitive to chlorine than fish are. A dose that leaves the fish visibly irritated but alive can still kill enough of the filter population to matter.

You will not see this on the day. It appears two to five days later as ammonia, then nitrite, in a tank that had been stable for months. Owners frequently treat that as a new and separate disaster.

If you have had any chlorine event, however mild:

Test ammonia and nitrite daily for a week.

Do small frequent water changes rather than one large one.

Keep aeration high.

Feed lightly, because uneaten food and waste is what the crippled filter cannot cope with.

Do not add fish, do not clean the filter, and do not replace media.

Do not dose bacterial supplements as a substitute for water changes; they can help, but the water changes are what keeps the fish alive while the population recovers.

The safe water change, step by step

Fill a clean container, used only for the aquarium, with cold water. Not hot, not mixed.

If the house has a softener, use a tap that bypasses it.

Note the volume against a marked line on the container.

Add the conditioner for that volume, at the full recommended rate, and stir.

Check the temperature against the tank by hand or with a thermometer, and adjust with more cold or with a heater rather than with hot tap water.

Aerate the container for a few minutes if you can, particularly with well water or water that has been standing.

Add slowly, ideally against the glass or onto a plate, so that no fish or plant gets a concentrated stream.

Watch the tank for the next hour.

For large tanks and ponds where buckets are impractical, use an inline conditioner unit with a cartridge you track and replace on schedule, and keep the flow within the rate the unit is rated for.

What never to do

Do not rely on standing water overnight if your supply uses chloramine.

Do not use the hot tap.

Do not use softened water from an ion-exchange softener.

Do not use reverse osmosis water without remineralising it.

Do not add conditioner to the tank and then run the tap straight in.

Do not dose at the minimum rate to make the bottle last.

Do not do a second large water change from the same source when the first one caused a problem.

Do not medicate a tank for a suspected parasite when the timing points at the water change.

Do not clean the filter in the same week as a chlorine event.

When to get help

Act immediately yourself, on the water, for a whole tank showing distress after a change. Conditioner, aeration, no feeding, lights off.

Contact an aquatic veterinarian or fish health service if fish are dying despite that, if gills are visibly damaged, or if you have a valuable or irreplaceable stock.

Contact your water supplier if you suspect a change in treatment, if the water is discoloured, or if there have been works in the street. They can usually tell you what has been done and when it will settle, and that information decides whether you should change water this week at all.

A newly filled tank is at its most dangerous in the first hour. Treat the water before it goes in and watch the fish afterwards.

Bring or record the tank volume, the volume changed, the conditioner brand and dose, whether the water came from a cold tap, a hose or a softened supply, the water test results before and after, and the timing of every sign relative to the change. In this problem the timeline is the diagnosis.